Well, in your collective wisdom you’ve guessed it, as I knew you would. These are in fact The Spandrels of San Marco (as one astute reader pointed out, they’re not really spandrels but pendentives) from the Basilica di San Marco in Venice. I visited the building not a week ago, and was marvelling at the architecture and its fantastic mosaics. As I was doing so, I recognized the spandrels (pendentives), each of which, as in my photograph of yesterday, contains a decoration. And I suddenly realized, “Holy shit, these are the spandrels of San Marco!” And so I photographed them.
They are famous for more than their mosaics: they are the ostensible topic of a famous paper by Steve Gould and Richard Lewontin (my Ph.D advisor), to wit: Gould, S. J., and R. C. Lewontin. 1979. The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist program. Proc. Roy. Soc. London B 205:581-598. The point of the article, made in its introduction given below, was that although spandrels harbor decorations, they were not put there by the architect as a surface to be decorated. Rather, spandrels (or pendentives, excuse me), are the necessary byproduct of placing a dome on top of arches and columns. And, like many features of organisms, spandrels are byproducts.
Gould and Lewontin pointed out that many traits of animals and plants were not the direct objects of natural selection, but reflect other processes. For example, blood is red not because it’s adaptive for blood to be red (I suppose an ardent sociobiologist could say that the red color makes blushes evident, which conveys emotions, etc. etc., but of course moles have red blood too!), but that the color is a byproduct of selection for an oxygen-carrying molecule, hemoglobin, that just happens to be red. In other words, it’s pleiotropy: a non-adaptive byproduct of an adaptation. In section 5 of their paper, G&L list many other ways that the traits of plants and animals can be nonadaptive.
This paper is famous because the authors were famous, because it’s very well written, but most of all because it posed a direct attack on the “Panglossian paradigm”: the view that sociobiology wants to explain all traits, particularly human behaviors, as the direct products of selection. This paper has been the subject of furious discussion and at least one book. In my view, the paper made some valid points but went overboard in its criticism of the adaptationist program, which, after all, has produced lots of insights about evolution. I knew Gould, who was on my thesis committee, and it always seemed like pulling teeth to get him to admit that natural selection was even a relatively important force in evolution. If pressed, he would, but Gould always preferred (perhaps for political reasons) to emphasize the limitations of selection. Lewontin was not nearly so extreme.
The opening is pure Gould:
The great central dome of St. Mark’s Cathedral in Venice presents in its mosaic design a detailed iconography expressing the mainstays of Christian faith. Three circles of figures radiate out from a central image of Christ: angels, disciples, and virtues. Each circle is divided into quadrants, even though the dome itself is radially symmetrical in structure. Each quadrant meets one of the four spandrels in the arches below the dome. Spandrels-the tapering triangular spaces formed by the intersection of two rounded arches at right angles are necessary architectural byproducts of mounting a dome on rounded arches. Each spandrel contains a design admirably fitted into its tapering space. An evangelist sits in the upper part flanked by the heavenly cities. Below, a man representing one of the four biblical rivers (Tigris, Euphrates, Indus, and Nile) pours water from a pitcher in the narrowing space below his feet.
(this image at http://www.bun.kyoto-u.ac.jp/ ~suchii/spandrel.html)
The design is so elaborate, harmonious, and purposeful that we are tempted to view it as the starting point of any analysis, as the cause in some sense of the surrounding architecture. But this would invert the proper path of analysis. The system begins with an architectural constraint: the necessary four spandrels and their tapering triangular form. They provide a space in which the mosaicists worked; they set the quadripartite symmetry of the dome above.
Such architectural constraints abound, and we find them easy to understand because we do not impose our biological biases upon them. Every fan-vaulted ceiling must have a series of open spaces along the midline of the vault, where the sides of the fans intersect between the pillars. Since the spaces must exist, they are often used for ingenious ornamental effect. In King’s College Chapel in Cambridge, for example, the spaces contain bosses alternately embellished with the Tudor rose and portcullis. In a sense, this design represents an “adaptation,” but the architectural constraint is clearly primary. The spaces arise as a necessary by-product of fan vaulting; their appropriate use is a secondary effect. Anyone who tried to argue that the structure exists be-cause the alternation of rose and portcullis makes so much sense in a Tudor chapel would be inviting the same ridicule that Voltaire heaped on Dr. Pangloss: “Things cannot be other than they are… Everything is made for the best purpose. Our noses were made to carry spectacles, so we have spectacles. Legs were clearly intended for breeches, and we wear them.” Yet evolutionary biologists, in their tendency to focus exclusively on immediate adaptation to local conditions, do tend to ignore architectural constraints and perform just such an inversion of explanation.
Read the rest; it’s certainly one of the most important papers in modern evolutionary biology.